April 5, 2007
Allvac titanium used for aerospace springs
The metastable beta titanium Allvac® 38-644 Alloy (Ti-38-644) has a long history of use for aerospace springs and aerospace fasteners. A vast range of mechanical properties is attainable for this alloy by manipulating the processing plan and adjusting subsequent thermal treatment. Ti-38-644 aerospace parts are typically processed in accordance with AMS 4958 for solution treated parts and AMS 4957 for cold drawn parts. An ongoing project at Allvac, an Allegheny Technologies Company, has recently focused on adjusting the processing and thermal treatment of Ti-38-644 from the AMS procedures to improve its viability and cost for higher volume markets. Particular attention has been given to cold drawing procedures and subsequent thermal treatments of 30 minutes or more. The cold working process may enable much shorter aging times than currently specified by AMS.
Metastable beta titanium alloys offer corrosion resistance, high strength and low elastic modulii thus making them excellent candidates for spring applications. In addition, the low density of these alloys makes them uniquely suited for weight reduction purposes. Companies such as Boeing, Lockheed and McDonald Douglas first started producing aircraft industry springs from Ti-38-644 in the late 1970s. Almost simultaneously, Ford initiated a project to study the use of Ti-38-644 for automotive suspension springs. The Ford study projected a 60% weight reduction associated with the transition from steel to titanium springs2.
Substantial opportunities for the cost reduction of beta titanium alloys for high volume applications lie in the areas of manufacturing and processing improvements. Some of these opportunities have been realized since the late 1970s when Ford conducted the initial study of Ti-38-644 for automotive suspension springs. In-line rotary forge (GFM) capability is now available at Allvac‘s high volume rolling mill3. The billet and subsequent coil weight which can be produced on Allvac‘c rolling mill is approximately three to four times larger than for current competitive producers. Other cost reduction procedures are in progress and still more are currently attainable as soon as the end users transition from sample orders to production size material orders. Larger ingots can be melted in sequence when volume demands. Allvac‘s high volume processing capability will allow for less set-up time, more efficient use of manpower and higher yields. In addition, the cold drawn product form, which is the focus of the investigation, reported herein, results in a more perfectly round bar or coil thus reducing finish conditioning and directly increasing yields.
The potential for cost reduction also exists at the spring manufacturing facilities. The Ti-38-644 material is shipped to the spring producers in a low strength condition for winding. After winding, the springs are aged to produce the high strength condition. The aging time that is required for solution treated and aged Ti-38-644 as verified by the Time-Temperature-Transformation diagram4 is typically greater than 12 hours. Specifically, AMS 4958 requires an aging time in the range of 6 to 20 hours. By cold drawing the material, nucleation of a second phase (HCP alpha) is aided during the aging process and aging times are reduced. AMS 4957, for cold drawn products, requires an aging time of 6 to 12 hours. The focus of this investigation was to explore the lower limits of aging time as a function of percent cold work and aging temperature. The successful implementation of shorter aging times could have the dual cost benefit of being less disruptive to the manufacturing cycle and allowing for the potential elimination of the post-process pickling procedure. Allvac successfully developed a new process and filed patent applications.
